I've categorized a number of photos and now want to write the category data to the file. I select the photos and then choose "Write Info to Files." When this is successful (that is, no errors appear) and I then open the file metadata I don't see that the categories were written as requested. Why is this?

Also, in some cases I get an error message "Cannot write information to <filename>." Can you clarify why I might be getting this error?

If I select an individual image, edit its Metadata and then choose to "Autofill IPTC Data" that seems to work correctly - but it is time consuming for 100s of photos.